Fluctuation analysis will be used to quantify the degree to which thyroid hormones and aldosterone stimulate the appearance of amiloride-sensitive Na+ channels in the apical membrane of the larval frog skin, during metamorphosis. In this technique, fluctuations in the ionic current across the epithelium are analyzed with a fast Fourier transform and the current through single channels (i) is calculated as is the density of channels in the membrane (N). This technique will also be applied to study K+ channels in the basolateral membrane of control and hormone - treated preparations. The basolateral membrane can be studied by reducing the apical membrane resistance with the ionophore, nystatin. It will thus be possible to determine whether stimulation of apical Na+ entry is coupled to an increase in i or N or basolateral K+ channels. Finally, impedance analysis will be made on the basolateral membrane so that its capacitance can be determined. From these measurements basolateral membrane area can be calculated and N expressed per unit of membrane area. The cellular mechanisms associated with the transport of Na+ by the frog skin are commonly used as a model for describing Na+ reabsorption by the mammalian nephron and colon. Studying factors which regulate the development of Na+ transport by the frog skin should thus further our understanding of the ontogeny and regulation of Na+ transport by the human kidney and colon.